This application is a national stage application filed under 35 U.S.C. 371 of International Application No. PCT/JP00/08596, filed Dec. 05, 2000.
1. Technical Field
The present invention relates to a method of producing an organic ruthenium compound for use in forming, through chemical vapor deposition, ruthenium thin film or ruthenium compound thin film.
2. Background Art
In recent years, thin film formed of a precious metal such as ruthenium, platinum, or iridium or thin film formed of an oxide thereof has been used as a material for forming an electrode in a capacitor included in ICs and LSIs. The reason for employment of these precious metals is that a thin film electrode produced from these precious metals is endowed with excellent electrode characteristics. Particularly, ruthenium and ruthenium compounds have become of interest in that these ruthenium species are expected to serve as main-stream materials for producing thin film electrodes.
Generally, ruthenium thin films and ruthenium compound thin films are produced through chemical vapor deposition (hereinafter referred to as CVD), because CVD facilitates production of thin film of uniform thickness and attains excellent step coverage (step covering performance). Therefore, CVD is expected to be a predominant process for producing a thin film electrode in the future, because CVD meets a demand of recent years for higher packaging density of circuits and electronic parts.
As source substances used in CVD, among metallic compounds, organometallic compounds are used in view of low melting point and ease of handling. Particularly, as an organic ruthenium compound, bis(cyclopentadienyl)ruthenium (ruthenocene) represented by Formula 1: 
has conventionally been used.
Bis(cyclopentadienyl)ruthenium, having high stability in air and no toxicity, is a suitable source for CVD. However, this compound is in solid form at ambient temperature and has a melting point as high as approximately 199-201xc2x0 C. Thus, a relatively large amount of energy is required for vaporizing the source.
In view of higher efficiency of thin film production, a variety of studies have been carried out on a ruthenium compound having a lower vaporization energy and lower melting point.
In order to lower the melting point of an organic ruthenium compound, there has been employed a method in which a functional group is added to at least one cyclopentadiene ring of bis(cyclopentadienyl)ruthenium, to thereby form a bis(cyclopentadienyl)ruthenium derivative.
Among such organic ruthenium compounds in which a functional group is added to a cyclopentadiene ring of bis(cyclopentadienyl)ruthenium, one promising candidate for a CVD source at present is bis(ethylcyclopentadienyl)ruthenium (also called 1,1xe2x80x2-diethylruthenocene), represented by Formula 2: 
in which the functional group is added to each cyclopentadiene ring.
Bis(ethylcyclopentadienyl)ruthenium is in liquid form at ambient temperature and has a relatively low melting point, to thereby provide sufficient vapor pressure. Thus, this compound is regarded a suitable CVD source substance endowed with essential characteristics.
In addition, an organometallic compound, represented by Formula 3: 
(wherein R2 represents a linear or branched alkyl group) in which a functional group is introduced into only one cyclopentadiene ring of bis(cyclopentadienyl)ruthenium also shows promise as a CVD source.
For example, (ethylcyclopentadienyl)cyclopentadienylrutheniumxe2x80x94R2 is an ethyl groupxe2x80x94has a melting point of approximately 12xc2x0 C., which is remarkably lower than that of bis(cyclopentadienyl)ruthenium. Thus, this ethyl-substituted compound is considered to serve as an excellent CVD source in a source vaporization step and a step of transferring the formed gas source.
As methods of producing these bis(cyclopentadienyl)ruthenium derivatives, the following methods are known.
The following three methods for introducing an ethyl group into each cyclopentadiene ring of bis(cyclopentadienyl)ruthenium; i.e., methods for producing bis(ethylcyclopentadienyl)ruthenium, are known.
The first method is drawn to a method of producing bis(ethylcyclopentadienyl)ruthenium by reducing bis(acetylcyclopentadienyl)ruthenium by sodium borhydride (NaBH4) (for detailed description of this production method, see G. B. Shul""pin, Zh. Obshch. Khim., vol. 51, 2152 (1981)).
The second method is drawn to a method of producing bis(ethylcyclopentadienyl)ruthenium through ligand-exchange reaction between bis(ethylcyclopentadienyl)iron ((C2H5C5H4)2Fe) and ruthenium trichloride (RuCl3) (for detailed description of this production method, see G. J. Gauthier, Chem. Commun., 690 (1969)).
The third method is drawn to a method of producing bis(ethylcyclopentadienyl) ruthenium by reacting ethylcyclopentadiene (C2H5C5H4) and ruthenium trichloride (RuCl3) in an alcoholic solvent in the presence of zinc powder (for detailed description of this production method, see Japanese Patent Application Laid-Open (Kokai) No. 11-35589).
Taking a method of producing (ethylcyclopentadienyl)cyclopentadienylruthenium as an example, a method of producing alkylcyclopentadienyl(cyclopentadienyl)ruthenium in which a functional group has been introduced into only one cyclopentadiene ring of bis(cyclopentadienyl)ruthenium is described next. In one known method, bis(cyclopentadienyl)ruthenium and acetic anhydride are reacted in the presence of aluminum chloride serving as a catalyst, to thereby form (acetylcyclopentadienyl)cyclopentadienylruthenium represented by the following Formula: 
(wherein R1 represents a linear or branched alkyl group) in which one hydrogen atom of bis(cyclopentadienyl)ruthenium is substituted by an acetyl group, and the thus-formed (acetylcyclopentadienyl)cyclopentadienylruthenium is reduced by aluminum chloride and lithium aluminum hydride (LiAlH4), to thereby form (ethylcyclopentadienyl)cyclopentadienylruthenium (for detailed description of this technique, see M. D. Rausch et al., J. Am. Chem. Soc., vol. 82, p76 (1960) and V. Mark et al., Inorg. Chem., vol. 3, No. 7, p1067 (1964)).
However, when a functional group is introduced into a cyclopentadiene ring of bis(cyclopentadienyl)ruthenium through any of these conventional methods, to thereby produce a derivative thereof, the below-described problems arise. (3) Problems Involved in Conventional Methods of Producing Bis(ethylcyclopentadienyl)ruthenium.
Problems in connection with the aforementioned three conventional methods of producing bis(ethylcyclopentadienyl)ruthenium is described. In the first method, sodiumxe2x80x94a component of sodium borohydride serving as a reducing agentxe2x80x94is intermingled as an impurity into produced bis(ethylcyclopentadienyl)ruthenium. As a result, sodium is also incorporated into the thin film prepared from this compound. Since an alkali metal such as sodium is an impurity which greatly affects electrical properties of the thin film, the bis(ethylcyclopentadienyl)ruthenium produced through the first method is not preferred as a CVD source substance.
The second method also involves a similar problem. Into bis(ethylcyclopentadienyl)ruthenium produced through the second method, an iron compound (ferrocene) having properties similar to those of bis(alkylcyclopentadienyl)ruthenium is intermingled. In addition, the iron compound is difficult to remove. Thus, when the bis(ethylcyclopentadienyl)ruthenium produced through the second method is used, a CVD apparatus and the thin film produced through CVD are also contaminated by the compound.
In terms of the purity of the product, the third method is more excellent that the other two methods. However, ethylcyclopentadiene which serves as a starting substance in this method is produced through pyrolysis of bis(ethylcyclopentadiene), which is generally difficult to obtain and is an expensive material. Thus, the produced bis(ethylcyclopentadienyl)ruthenium also becomes expensive, disadvantageously causing an increase in cost of semiconductor products. In addition, ethylcyclopentadiene has poor stability and is readily dimerized when allowed to stand at room temperature, to thereby form bis(ethylcyclopentadiene). Thus, difficulty in handling of ethylcyclopentadiene is problematic during production steps. (4) Problems Involved in Conventional Methods of Producing Alkylcyclopentadienyl(cyclopentadienyl)ruthenium
In a conventional method of producing alkylcyclopentadienyl(cyclopentadienyl)ruthenium, reactants are contaminated with aluminum originating from aluminum chloride serving as a catalyst and lithium aluminum hydride serving as a reducing agent, to thereby disadvantageously lower the purity of the produced (ethylcyclopentadienyl)cyclopentadienylruthenium. When (ethylcyclopentadienyl)cyclopentadienylruthenium of such a low purity is used to form thin film, aluminum serving as an impurity is intermingled into the formed thin film, thereby possibly deteriorating the electrical characteristics of the thin film. In addition, use of a low-purity source may cause contamination of a CVD apparatus.
Furthermore, aluminum chloride, which is used in both reaction steps, has poor stability and readily decomposes in air, to thereby produce hydrochloric acid gas. Accordingly, employment of the aforementioned known method may cause corrosion of an apparatus for producing (ethylcyclopentadienyl)cyclopentadienylruthenium, and a special means for anti-corroding is required, to thereby possibly elevate apparatus costs.
As mentioned above, conventional methods of producing bis(alkylcyclopentadienyl)ruthenium or alkylcyclopentadienyl(cyclopentadienyl)ruthenium raise problems in terms of product purity and production costs of target compounds.
The present invention has been accomplished in order to solve the aforementioned problems. Thus, an object of the present invention is to provide a method for producing bis(ethylcyclopentadienyl)ruthenium or alkylcyclopentadienyl(cyclopentadienyl)rutheniumxe2x80x94an organic ruthenium compound which can be employed as a CVD sourcexe2x80x94which method can produce such a compound at considerably high purity and low cost.
On the basis of the fact that conventionally known methods of producing (alkylcyclopentadienyl)cyclopentadienylruthenium are limited only to production of (ethylcyclopentadienyl)cyclopentadienylruthenium, another object of the present invention is to provide a method of producing (alkylcyclopentadienyl)cyclopentadienylruthenium in which an alkyl group other than an ethyl group has been introduced, and to elucidate properties of the compound.
The present inventors have carried out extensive studies, and have found that a novel method of producing bis(ethylcyclopentadienyl)ruthenium or alkylcyclopentadienyl(cyclopentadienyl)ruthenium, which method differs from similar, conventional methods.
The methods of producing the respective organic ruthenium compounds is described next in detail.
The method, which the present inventors hereby disclose, of producing bis(ethylcyclopentadienyl)ruthenium is drawn to a method, as recited in claim 1, of producing bis(ethylcyclopentadienyl)ruthenium represented by the below-described Formula 6: 
comprising hydrogenating bis(acetylcyclopentadienyl)ruthenium represented by the below-described Formula 5: 
in the presence of a catalyst.
Claim 1 of the present invention provides a method of producing bis(ethylcyclopentadienyl)ruthenium, characterized by hydrogenating bis(acetylcyclopentadienyl)ruthenium, which is also employed as a starting material in the aforementioned first conventional method. According to the present invention, high-purity bis(ethylcyclopentadienyl)ruthenium can be produced, since the reaction system contains no latent impurity element; e.g., iron or an alkali metal such as sodium.
In the present invention, bis(acetylcyclopentadienyl)ruthenium serving as a starting material for producing bis(ethylcyclopentadienyl)ruthenium is a compound which can be produced readily at low cost. Thus, according to the present invention, bis(ethylcyclopentadienyl)ruthenium can be produced at low cost without use of expensive bis(ethylcyclopentadiene), which is employed in the aforementioned third conventional method.
The expression xe2x80x9cin the presence of a catalystxe2x80x9d recited in claim 1 is based on the fact that hydrogenation of bis(acetylcyclopentadienyl)ruthenium of the present invention can be effected essentially in the presence of a catalyst. As recited in claim 2 of the present invention, catalysts such as a platinum catalyst, a palladium catalyst, a ruthenium catalyst, and the Raney nickel catalyst are preferably used. Examples of particularly preferred platinum catalysts include a platinum-carbon catalyst and a platinum oxide catalyst (the Adams catalyst).
The hydrogenation is carried out preferably under the following conditions: reaction temperature of room temperature to 150xc2x0 C. and hydrogen pressure of 1xc3x97105 to 5xc3x97106 Pa.
As recited in claim 3, bis(acetylcyclopentadienyl)ruthenium is preferably obtained by reacting the bis(cyclopentadienyl)ruthenium represented by the below-described Formula 7: 
with acetic anhydride in the presence of a phosphoric acid catalyst.
As the catalyst, aluminum chloride (AlCl3) could also be used. However, aluminum chloride is highly decomposable in air and generates hydrochloric acid gas, which is not preferable in operational circumstances and corrodes the reaction apparatus.
During production of bis(acetylcyclopentadienyl)ruthenium from bis(cyclopentadienyl)ruthenium, the ratio of the amount by mol of bis(cyclopentadienyl)ruthenium to that of acetic anhydride during reaction is preferably 1:2. When the amount of acetic anhydride is comparatively low, (acetylcyclopentadienyl)cyclopentadienylruthenium is intermingled into reaction products, thereby lowering the yield of bis(acetycyclopentadienyl)ruthenium from bis(cyclopentadienyl)ruthenium.
As recited in claim 4, the bis(cyclopentadienyl)ruthenium which is obtained by reacting cyclopentadiene and ruthenium chloride with zinc powder is preferably used. Cyclopentadiene is readily produced by pyrolizing dicyclopentadiene, which is contained in a large amount in crude benzene derived from tar and in cracked naphtha and is available in large amounts at low cost. Thus, according to the present invention, ruthenocene, furthermore bis(ethylcyclopentadienyl)ruthenium can be produced at low cost by use of cyclopentadiene as a starting material.
The method of producing alkylcyclopentadienyl(cyclopentadienyl)ruthenium according to the present invention is described next. The method, which the present inventors hereby disclose, of producing alkylcyclopentadienyl(cyclopentadienyl)ruthenium is, drawn the method according to claim 5, in which bis(cyclopentadienyl)ruthenium represented by Formula 8: 
is acylated with carboxylic anhydride represented by the formula shown below:
(R1CO)2O xe2x80x83xe2x80x83Formula 9
(wherein R1 represents a linear or branched alkyl group) in the presence of phosphoric acid serving as a catalyst, to thereby form (acylcyclopentadienyl)cyclopentadienylruthenium represented by the formula shown below: 
(wherein R1 has the same meaning as mentioned above), and the thus-formed (acylcyclopentadienyl)cyclopentadienylruthenium is reduced, to thereby form (alkylcyclopentadienyl)cyclopentadienylruthenium represented by the formula shown below: 
(wherein R2 represents a linear or branched alkyl group).
Claim 5 of the present invention is based on the present inventors"" finding that bis(cyclopentadienyl)ruthenium can readily be acylated by reaction with carboxylic anhydride in the presence of phosphoric acid serving as a catalyst. According to the present invention, high-purity (acylcyclopentadienyl)cyclopentadienylruthenium can be obtained, since aluminum chloride, which may generate an impurity, is not employed as a catalyst.
In claim 5 of the present invention, the (acylcyclopentadienyl) cyclopentadienylruthenium produced in the presence of a phosphoric acid catalyst is reduced, to thereby form (alkylcyclopentadienyl)cyclopentadienylruthenium. The term xe2x80x9creductionxe2x80x9d in connection with claim 5 refers, in a broad sense, to a process for combining a compound with hydrogen to form a novel compound. However, in a manner similar to that employed in the aforementioned conventional method in which reduction is defined in a narrow sense, (alkylcyclopentadienyl)cyclopentadienylruthenium may be produced through reduction by use of aluminum chloride and lithium aluminum hydride. The criteria for employment of reduction by use of these reagents are that the (acylcyclopentadienyl)cyclopentadienylruthenium produced according to the present invention has high purity and that the purity of (alkylcyclopentadienyl)cyclopentadienylruthenium produced from the acyl compound is higher than that of (alkylcyclopentadienyl)cyclopentadienylruthenium produced through the conventional method.
However, in order to produce (alkylcyclopentadienyl)cyclopentadienylruthenium of higher purity, (acylcyclopentadienyl)cyclopentadienylruthenium is preferably reduced through hydrogenation as recited in claim 2. The reason for the employment of hydrogenation is that no reducing agent, such as aluminum chloride or lithium aluminum hydride, which possibly causes contamination is used, to thereby produce high-purity (alkylcyclopentadienyl)cyclopentadienylruthenium.
As described above, the present invention is characterized by enhancing the purity of products both in a step for converting bis(cyclopentadienyl)ruthenium to (acylcyclopentadienyl)cyclopentadienylruthenium and a step for converting (acylcyclopentadienyl)cyclopentadienylruthenium to (alkylcyclopentadienyl)cyclopentadienylruthenium.
The substituent R2 of the produced (alkylcyclopentadienyl)cyclopentadienylruthenium can be formed into a desired substituent by selecting carboxylic anhydride to be reacted with bis(cyclopentadienyl)ruthenium. Some specific examples are shown in Table 1.
During the above reaction, the ratio of the amount by mol of bis(cyclopentadienyl)ruthenium to that of carboxylic anhydride is preferably 1:1; i.e., equivalent. When the amount of carboxylic anhydride is comparatively small, the yield of (acylcyclopentadienyl)cyclopentadienylruthenium decreases, whereas when the amount of carboxylic anhydride is comparatively large, bis(acylcyclopentadienyl)rutheniumxe2x80x94species in which two cyclopentadiene rings of bis(cyclopentadienyl)ruthenium are substitutedxe2x80x94is simultaneously formed. In this case, the yield of (acylcyclopentadienyl)cyclopentadienylruthlenium also decreases.
In claim 5 of the present invention, (acylcyclopentadienyl)cyclopentadienylruthenium is formed through hydrogenation in a manner similar to that of claim 1. Accordingly, claim 5 also includes an expression xe2x80x9cin the presence of a catalyst,xe2x80x9d to thereby clearly describe the requirement of the presence of a catalyst. As recited in claim 8, a catalyst such as a platinum catalyst, a palladium catalyst, a ruthenium catalyst, or the Raney nickel catalyst is preferably used. Examples of particularly preferred platinum catalysts include a platinum-carbon catalyst and a platinum oxide catalyst (the Adams catalyst).
In addition, hydrogenation is carried out preferably under the following conditions: reaction temperature of room temperature to 150xc2x0 C. and hydrogen pressure of 1xc3x97105 to 5xc3x97106 Pa.
As described above, the species of (alkylcyclopentadienyl)cyclopentadienylruthenium produced through the method according to the present invention have high purity and contain no impurity such as aluminum. In addition, these compounds are in liquid form or solid form of low melting point at room temperature and have a high vapor pressure at approximately 100xc2x0 C. Thus, these compounds are suitable for CVD sources.
Regarding each organic ruthenium compound, suitable modes for carrying out the present invention is described next.
Dicyclopentadiene (2000 g) was pyrolized at 180xc2x0 C., to thereby form cyclopentadiene, and the thus-formed cyclopentadiene was purified through distillation at 40xc2x0 C. To the purified cyclopentadiene (1500 g), ruthenium trichloride (130 g) and ethyl alcohol (2500 ml) were added, and the resultant reaction mixture was cooled to xe2x88x9210xc2x0 C. After cooling was complete, zinc powder (163 g) was added to the reaction mixture in a divided (7 portions) manner at uniform intervals so as to allow the mixture to react. The resultant reaction mixture was subjected to extraction by use of benzene, and the product was recrystallized in hexane, to thereby yield 80 g of bis(cyclopentadienyl)ruthenium.
The thus-obtained bis(cyclopentadienyl)ruthenium (3 g), acetic anhydride (20 ml), and 85% phosphoric acid (2.0 ml) were placed in a round-bottom flask, and the mixture was heated at 85xc2x0 C. for one hour. Subsequently, the reaction product was neutralized by use of sodium hydroxide. The neutralized mixture was subjected to extraction by use of hexane, thereby separating bis(acetylcyclopentadienyl)ruthenium.
The thus-produced bis(acetylcyclopentadienyl)ruthenium (10.8 g) was weighed and dissolved in methanol (300 cc), and a 5% Pd/C catalyst (1 g) was added to the resultant solution. The mixture was allowed to react at a hydrogen pressure of 3.45xc3x97105 Pa (50 psi) for 24 hours. The resultant reaction mixture was purified through distillation at 100xc2x0 C. and 3.99xc3x9710xe2x88x923 Pa (0.3 torr), to thereby yield 5.7 g of bis(ethylcyclopentadienyl)ruthenium. The bis(ethylcyclopentadienyl)ruthenium was found to have a melting point of 6xc2x0 C.
The procedure of Embodiment 1 was repeated, except that a 10% Pt/C catalyst was used instead of the 5% Pd/C catalyst, to thereby produce bis(ethylcyclopentadienyl)ruthenium.
The procedure of Embodiment 1 was repeated, except that a platinum oxide catalyst was used instead of the 5% Pd/C catalyst, to thereby produce bis(ethylcyclopentadienyl)ruthenium.
The procedure of Embodiment 1 was repeated, except that the Raney nickel catalyst was used instead of the 5% Pd/C catalyst, to thereby produce bis(ethylcyclopentadienyl)ruthenium.